Signal processing apparatus including doppler dispersion correction means

ABSTRACT

Apparatus for processing data signals such as sonar return signals of the frequency modulated type wherein adverse effects of Doppler dispersion are eliminated which apparatus includes a signal generator providing an inverse replica of that signal which is shifted upwardly in frequency and is transmitted as the sonar signal, a pair of digital time compression units for compressing respective ones of the down-shifted sonar return signal, i.e., a data signal, and the replica signal, a pair of step clock generators each enabling the memory bank contents of the associated time compression unit to be read out at respective various rates thereby varying in steps the time compression ratios of the units and bandpass correlation apparatus including a first balanced modulator for receiving and operating on the compressed output signals of the time compression unit. The output frequencies of the step clock generators vary in steps so that the compressed replica signal frequency is, in effect, translated upwardly by decreasing degrees for a range of predetermined positive relative target velocities and, thereafter, the compressed data signal frequency is translated upwardly by increasing degrees for a range of predetermined negative relative target velocities thereby enabling, in effect, correlation of the data signal with a suitable replica signal for each of a number of predetermined relative target velocities in a predetermined range. The filtered output signal from the first modulator and an output signal of a step oscillator whose output signal frequency varies in steps according to the predetermined relative target velocities are applied to a second modulator to compensate for Doppler-induced frequency shifts in the transmitted sonar signal. The filtered output signal of the second modulator is detected and integrated to provide points on the correlation functions in the range-Doppler plane.

United States Patent Jacobson et a1.

[ 1 Mar. 19, 1974 SIGNAL PROCESSING APPARATUS INCLUDING DOPPLERDISPERSION CORRECTION MEANS [75] Inventors: Arthur J. Jacobson, FortWayne, 1nd.; Gilbert J. Huey, San Diego, Calif.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

[22] Filed: Sept. 12, 1968 [21] App]. No.: 761,010

52 US. Cl. FtO/ZFM, 340/3 D,

343/5 DP, 343/9, 343/100 CL [51] Int. Cl. G015 9/68 [58] Field of Search340/3, 3 FM, 3 D; 343/14,

343/17.2, 17.2 PC, 100.7, 50 P, 8, 9

[56] References Cited UNITED STATES PATENTS 2,834,956 5/1958 Harris, Jr.343/14 3,130,404 4/1964 Fried 343/14 3,199,106 8/1965 Karr 343/l7.2

3,355,579 11/1967 Robertson.... 343/1007 3,404,400 10/1968 Miller, Jr.340/3 X Primary ExaminerRichard A. Farley I Attorney, Agent, orFirm-Edgar .I. Brower; Henry 1 Hansen; B. Frederick Buchan, Jr.

[57] ABSTRACT our RECEIVER eliminated which apparatus includes a signalgenerator providing an inverse replica of that signal which is shiftedupwardly in frequency and is transmitted as the sonar signal, a pair ofdigital time compression units for compressing respective ones of thedown-shifted sonar return signal, i.e., a data signal, and the replicasignal, a pair of step clock generators each enabling the memory bankcontents of the associated time compression unit to be read out atrespective various rates thereby varying in steps the time compressionratios of the units and bandpass correlation apparatus including a firstbalanced modulator for receiving and operating on the compressed outputsignals of the time compression unit. The output frequencies of the stepclock generators vary in steps so that the compressed replica signalfrequency is, in effect, translated upwardly by decreasing degrees for arange of predetermined positive relative target velocities and,thereafter, the compressed data signal frequency is translated upwardlyby increasing degrees for a range of predetermined negative relativetarget velocities thereby enabling, in effect, correlation of the datasignal with a suitable replica signal for each of a number ofpredetermined relative target velocities in a predetermined range. Thefiltered output signal from the first modulator and an output signal ofa step oscillator whose output signal frequency varies in stepsaccording to the predetermined relative target velocities are applied toa second modulator to compensate for Doppler-induced frequency shifts inthe transmitted sonar signal. The filtered output signal of the secondmodulator is detected and integrated to provide points on thecorrelation functions in the range-Doppler plane.

17 Claims, 5 Drawing Figures snow.

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DISPLAY 57 INVENTORS ARTHUR J. JACOBSON BANDPASS DETECTOR GILBERT J.HUEY FILTER a BY INTEGRATOR wga h ATTORNE s SIGNAL PROCESSING APPARATUSINCLUDING DOPPLER DISPERSION CORRECTION MEANS BACKGROUND OF THEINVENTION This invention relates generally to signal processingapparatus and, more particularly, to sonar return signal processingapparatus of the bandpass correlation type.

Detection of return sonar signals under conditions wherein a relativelylow signal-to-noise ratio applies may be accomplished by correlating thereturn sonar signal with a replica thereof. Also, the effective range ofsonar apparatus can be increased by utilizing known techniques ofproviding frequency modulated signals and effecting pulse compression.Using matched filters to detect sonar return signals from moving targetsis greatly complicated by the effects of Doppler dispersion upon sonarsignals of varying frequency because the portions of the signal havingthe greatest frequency experience the greatest Doppler shift. Theambiguity or uncertainty function X('r, 4)) provides a convenient meansof describing the effects of signal modulation in terms of range andDoppler resolution, of reverberation, of discrimination, and of rangeand Doppler ambiguities and may be defined by:

wherein:

1' time delay, (I) Doppler frequency shift, p.(t) the signal as afunction of time (waveform),

U(j) the signal as a function of frequency (spectrum) and denotes thecomplex conjugate. Because equation 1 treats the Doppler frequency shiftas being uniform throughout the modulation bandwidth W, its applicationcan lead to erroneous conclusions particularly in sonar applicationswhere the bandwidth-time products are not small in comparison to theDoppler factor c/2v where c is the velocity of propagation and v is therelative target velocity. Doppler dispersion thereby distorts thefrequencytime characteristic of the signal, degrades the degree of timeresolution achievable by the processing apparatus and can madecorrelation most difficult. The provision of a plurality of signalgenerators, each generating a suitable replica of the anticipated returnsignal which has undergone the appropriate Doppler frequency shifts overits entire bandwidth attributable to a corresponding one of severalpredetermined relative target velocities undesirably increases the cost,size and complexity of the processing apparatus.

SUMMARY OF THE INVENTION It is the general purpose of this invention toprovide signal processing apparatus wherein the undesirable effects ofDoppler dispersion are eliminated and wherein the return signal of atransmitted sonar or radar signal of the frequency modulated type may beprocessed in a manner to assure detection with a high degree ofresolution. Briefly, the general purpose of the invention may beaccomplished by providing apparatus to make a correction for Dopplerdispersion. More particularly, the general purpose of the invention isaccomplished by providing processing apparatus including a single signalgenerator providing a replica signal and a pair of digital timecompression units whose time compressed output signals are read out forcorrelation by correlation apparatus at various rates calculated toeliminate the adverse effects of Doppler dispersion on data signals ofvarying frequency. The invention further contemplates modulating a sumsignal derived from the time compressed data and replica signals withthe output signal of a step oscillator having an output frequencyvarying in steps corresponding to predetermined relative targetvelocities in order to compensate for the actual Doppler frequency shiftintroduced into the received signal. The invention additionallycontemplates that the sonar return signal be down modulated prior todispersion correction to a frequency range suitable for sampling at arate low enough relative to a usable read out rate in order to provide asatisfactorily high time compression ratio.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1a and lb representfrequency-time characteristics of various transmitted sonar signals, thereturns thereof which have Doppler frequency shifts and suitable replicasignals;

FIG. 2 represents a block diagram of signal processing apparatusaccording to the invention;

FIG. 3 represents a block diagram of a time compression unit included inthe apparatus of FIG. 2;

FIG. 4 represents a time diagram of the wave form envelopes of thefrequency stepped output signals of step clock generators associatedwith respective time compression units of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention may be betterunderstood by referring to the frequency-time characteristics of FIGS.la and 1b wherein the characteristics A and A represent frequency-timecharacteristics of signals to be shifted upwardly in frequency fortransmission and having respectively increasing and decreasing changesin frequency over a predetermined bandwidth W during a predeterminedperiod of signal duration T. The characteristics A, and A, have centerfrequencies selected to provide manageable signals when compressed by apredetermined desired compression ratio. The characteristics B and Brepresent those of the sonar signals to be transmitted in a desiredfrequency range and also the returns thereof from relatively stationarytargets. The slopes of A A B and B are identical in magnitude, therebeing a constant difference frequency between A and B and between A andB The characteristics C and C represent the frequency-timecharacteristics of sonar return signals which have undergone anadditional upward shift in frequency by reason of the Doppler effectduring positive relative target velocity conditions, i.e., the target isapproaching the signal transmitting-receiving point. Similarly,characteristics D and D respectively, represent the frequencytirnecharacteristics of return data signals reflected from the target havinga negative relative velocity, i.e., the target is moving away from thetransmittingreceiving point. As shown in exaggeration, the Dopplereffect not only causes unequal shifts in frequency as viewed at the endsof the frequency band of the signal but also causes slight compressionor expansion of the duration of the sonar return signal.

The characteristics E, and E indicated by dashed lines represent thefrequency-time characteristics of the replica signals to be applied tothe correlation apparatus of FIG. 2 along with the respective datasignals, i.e., return signals which have been down modulated by theabove-mentioned constant difference frequency. The slopes of E and E,are equal in magnitude but opposite in sign to those of A, and A and thecenter frequencies and bandwidths W of E, and A, or of E and A are thesame.

The Doppler-shifted frequencies f,, at the lower end of the frequencyband of the return signals, may be defined as:

f =(1+ 2v/c) (f W/2) (Equation 2) wherein f is the center frequency ofthe transmitted band and wherein the term v /c is disregarded because ofits relative insignificance. The Doppler frequenciesf at the upper endsof the received bands may be defined as:

f (l 2v/C) (f W/2) (Equation 3) From equations 2 and 3, it can be shownthat the Doppler dispersion Af, which is representative of the change infrequency slope across the bandwidth and which may be defined by thedifference between the doppler shift at the upper and that at the lowerends of the received band may be expressed by:

(Equation 4) Thus, the Doppler dispersion, Af, is independent of thecenter frequency f The effect of time compression or expansion must alsobe taken into account particularly in the case of signals characterizedby linear frequency modulation. Since one criteria for processing thereturn signal is that the dispersion Af must be less than the effectivepost-correlation bandwidth, which, in turn, is equivalent to the inverseof the duration T of the signal, the bandwidth-time duration product WTmust be less than the Doppler factor c/2v.

In general, the apparatus of FIG. 2 functions to correlate the downmodulated sonar return signal, hereinafter referred to as the datasignal, with the replica signal. The adverse effects of Dopplerdispersion are eliminated, in effect, by readjusting the magnitude ofthe slope of one of the data signal frequency-time characteristics orthe replica signal frequency-time characteristics to approximate that ofthe other by relatively varying time compression ratios of the replicasignal and the data signal in a series of steps each calculated toachieve a common slope magnitude for the corresponding predeterminedrelative target velocity in a predetermined range. Thereafter, theapparatus functions, in effect, to correct for the center frequencyshift of the sonar return signal attributable to the Doppler effect forthe corresponding predetermined relative target velocity to providesignals indicative of points on the correlation functions in therange-Doppler plane for each of the predetermined relative velocities.Of course, the output signal of maximum amplitude exceeding a thresholddetection level and indicative of a point on a particular correlationfunction may be associated with the target range and the correspondingrelative velocity and may be displayed accordingly.

Referring now to the apparatus of FIG. 2, at some time, as when it isdesired that a sonar signal be transmitted, a control unit 10 provides asignal to a matched pair of FM signal generators 11 causing them toprovide frequency modulated signals having frequencytime characteristicssuch as A, and E, or A and E which have slopes equal in magnitude andopposite in sign. The generators 11 may alternatively be of a typeincluding a single frequency modulated signal generator and apparatusfor storing the signal supplied and reading it out in inverse order. Thesignal having a characteristic such as A, is fed to a mixer 12 which, inturn, receives the single frequency output signal of one of theoscillators l3 controlled by the unit 10 and provides its sum outputsignal having the characteristic such as B, and having an upwardlyshifted center frequency in a desired range to a conventional sonarsignal transmitter 14. The apparatus also includes a data receiver 15which is arranged to receive the echoed sonar return signal directly orvia a communication link as desired. The output signal of the receiver15 is applied through a noise eliminating bandpass filter 16 to a mixer17 which, in turn, is connected to receive the output signal of the sameone of the oscillators 13 for translating the received signal downwardlyin frequency. The output signal of the mixer 17 is applied through abandpass filter 18 to a signal selector 19 and comprises the data signalwhose frequency band is in a range enabling sampling at a sufficientlylow rate to permit processing or compressed signals having frequenciesof the magnitude achieved by multiplying the down shifted data signalfrequencies by a desired compression ratio. The signal selector is alsoconnected to receive a replica signal from the generators 11 whosefrequency-time characteristic such as E, has a slope equal in magnitudeand opposite in sign to that of the signal such as A, supplied to themixer 12.

The signal selector 19 is controlled by the unit 10 and normally appliesthe data signal to a clipper amplifier 22 except during the period it isdesired to apply the replica signal such as E, from the generators 11 tothe clipper amplifier 22 as when a sonar signal is being transmitted oralternately on a time-sharing basis. The output of the clipper amplifier22 is fed to a sampler 23 which, in turn, supplies a correspondingoutput signal comprising a series of digital words at a sample ratecontrolled by a sample rate oscillator 24. The sample rate oscillator24, in turn, is also controlled by the control unit 10. One suitableform for the output signal of the sampler may comprise a digital wordONE when, at the time a sample is made, the output signal of the clipperamplifier 22 is positive and a digital word ZERO when the amplifiersignal is negative. A suitable relationship for the sample rate exceedsas by a factor of three, the maximum anticipated doppler shiftedfrequency component of the data signal being applied to the signalselector 19. For example, for a desired compression ratio on the orderof 2432 and a down shifted data signal having a center frequency on theorder of 800 Hz and having a bandwidth on the order of plus or minus 200Hz, a sample rate on the order of 4,096 Hz has been found to besatisfactory.

The serial digital signal from the sampler 23 is applied through a pairof gates 26 and 27, controlled by the control unit 10, to data signaland replica signal digital time compression units 30 and 31 wherein thedata and replica signals are respectively stored and read out at muchgreater rates than the sample rate thereby effecting time compression ofthe data and replica signals. Of course, the sample rate oscillator 24,the gates 26 and 27 and the signal selector 19 can be controlled orsequenced by the control unit in a conventional manner so thatappropriate samples of both the data signal and the replica signal arealternately supplied at the same sample rate to their respectivecompression units 30 and 31 even though the transmission time of a sonarsignal of a selected duration and the time of reception of the returnsignal may overlap. Additionally, it is contemplated that once a replicasignal has been provided by the generators 11 and converted to digitalform and stored in the unit 31 successive sampled replicas of eachsucceeding transmitted signal need not necessarily be sampled andstored.

The unit 30 generally compresses the input signal thereto in the samemanner as the electronic time compression device disclosed in US. Pat.No. 3,295,107 issued Dec. 27, 1966, to R. E. Stalcup. The compressionratio CR for a unit is defined as the ratio of the input signal readoutrate to the input signal sample rate. The data signal time compressionunit 30, disclosed in more detail in FIG. 3, generally includes aregister 33 into which are loaded a number of succeeding bits ofinformation from the sampler 23. When the register 33 has been filled,its contents are applied in parallel by gates 34 controlled by the unit10 for loading into a memory bank 36. The memory bank 36 has at least asufficient capacity to store a number of bits collectively representingin real time the duration T, such as one second, of the transmittedsonar signal. As the newest bits of information are loaded into thememory bank 36, a corresponding number of the oldest bits are discarded.The unit 30 further includes read control circuitry 37 whichsuccessively fills a readout shift register 38 with copies of thecontents of succeeding groups of storage elements in the memory bank 36,and thereby enables reading out the entire contents of the memory bank36 in order. The shift register 38 responds to clock pulses appliedthereto from a data readout-rate step clock generator 39 to shift outits contents in series of digital signals collectively indicative of thesignal applied to the signal selector 19 as compressed in time.

Referring again to FIG. 2, the replica signal time .compression unit 31is controlled by the unit 10 and similarly receives and stores in itsmemory bank the replica signal which has been sampled at the same samplerate as the data signal under the control of the oscillator 24. The unit31 is read out at a rate controlled by the replica read-out rate stepclock generator 40. The apparatus in this manner provides timecompressed data and replica signals which, in effect, are compared bycorrelation to ascertain the degree of similarity thereof.

As indicated above by equations 2 and 3, each fre quency component ofthe return signal will have undergone a shift in frequency in accordancewith the Doppler effect which shift is a function of the transmittedcomponent frequency and the relative target velocity. The effect of timecompressing the data and replica signals can be visualized asmulitplying each frequency component of each signal by the timecompression ratio effected by the respective unit 30 or 31. Thecompression ratio CR of the replica signal unit is made different fromthe compression ratio CR of the data signal unit in accordance with thefollowing equation:

CR (l 2v/c) CR (Equation 5) Thus, by varying the compression ratio ofthe replica unit 31 in a series of steps in accordance with equation (5)each corresponding to a particular relative target velocity v in adesired range, one may effectively adjust the magnitude of the slope ofthe frequency-time characteristic of the replica signal to equal that ofthe data signal and thereby correct for Doppler dispersion.

The velocity difference between velocities corresponding to successivecompression ratio steps in dependent upon the desired degree ofresolution of the apparatus. For example, it has been found thatestablishing compression ratios corresponding to steps of four knots inrelative target velocity will enable the provision of a possiblydetectable point on at least one of the correlation functions associatedwith respective four-knot steps for all relative target velocities inthe design range The use of steps in excess of four knots has been foundto cause unacceptably low correlation function peak magnitudes achievedunder relative target velocity conditions intermediate the designedvelocity steps.

While equation (5) holds true for both positive and negative relativetarget velocities, it is desirable that the compression ratio for eitherunit 30 or 31 never drop below a predetermined minimum. Since fornegative target velocities in accordance with equation (5) CR will beless than CR appropriate values for the compression ratio CR of the datasignal in the case of negative relative target velocities in terms ofthe minimum desired replica compression ratio CR may be established inaccordance with the following relationship:

CR CR /(l 2v/c) (Equation 6) In accordance with either equations (5) or(6), the compression ratios of the units 30 and 31 are equal where therelative target velocity is zero.

Since the sample rates of the input signals to both units 30 and 31 arethe same, the resepctive readout clock frequencies of the units 30 and31 for each step corresponding to a given velocity v are related in thesame manner as are the respective compression ratios of equations (5)and (6). Accordingly, the data and replica readout-rate step clockgenerators 39 and 40 are each mechanized to provide for each stepcorresponding to a particular predetermined relative target velocity v asufficient number of clock pulses to cause the entire contents of theirrespective memory banks to be read out at the appropriate frequency. Tosynchronize the operation of the clock generators 39 and 40 a step-rateoscillator 41 controlled by the unit 10 provides a series of pulses eachcausing the generators 39 and 40 to provide their output signals havingthe respective appropriate frequencies for each corresponding relativevelocity step. In this manner, the data signal which has been sampledand stored in the unit 30 may be correlated with the replica signal asthe relative slopes of the frequency-time characteristics of the dataand replica signals are relatively adjusted in a series of steps eachcorresponding to a predetermined relative target velocity in a desiredrange. New information which is being stored in the register 33 of theunit 30 can be entered into the memory bank 36 after the series ofvelocity steps in a correlation cycle has been completed and before thenext correlation cycle begins. Some degree of similarity between thesignals should be achieved during at least one of the steps in somecorrelation cycle. A maximum similarity indication, of course, pinpointsthe time of the return of the sonar signal echo.

FIG. 4 illustrates suitable envelopes F and G of the series of clockpulses provided by the generators 39 and 40 for a compression cycleincluding 19 steps. At time 1 the step-rate clock generator 41 causesboth the generators 39 and 40 to begin emitting clock pulses at theappropriate frequencies for the step corresponding to the largestpositive relative target velocity in the designed range. Since thereadout frequency of the unit 31 is greater than the readout frequencyof the unit 30 in accordance with the relationship derived from equationthe entire contents of the memory bank of the unit 31 will be read outbefore the contents of that of the data unit 30. Thereafter, at time twhen the contents of the memory bank 36 of the data unit 30 have beenread out, the step-rate clock generator 41 causes the block generators39 and 40 to begin emitting the required number of clock pulses at theappropriate frequencies corresponding to the next smaller relativetarget velocity. At time 1 for the step corresponding to a relativetarget velocity of zero, the frequencies as the clock provided by thegenerators 39 and 40 are the same. Therefore, the clock pulse envelopefor each generator 39 and 40 has the same duration. At time t for thestep corresponding to the greatest negative relative target velocity inthe designed range, the contents of the memory bank 36 of the data unit30 can be read out by the clock pulses provided by the generator 39 in alesser amount of time than is required to read out the memory bankcontents of the replica unit 31. The control unit 10, therefore,provides at time 1 a data readin signal H to the gates 34 in the unit sothat the latest occurring information in the data signal which is beingstored in register 33 can be entered into the memory bank 36 and theoldest information therein discarded. Thereafter, at time t when thememory bank contents of unit 31 have been read out, a new correlationcycle begins.

The sample rate, the minimum desired compression ratio, the number ofwords of information to be loaded in a correlation cycle, and the numberof correlation cycles per second are all interrelated. For example, theproduct of the sample rate, e.g., 4096, and the minimum desiredcompression ratio, e.g., 2432, indicates the minimum number of bits ofinformation read out per second, i.e., the read out frequency for thestep corresponding to a relative target velocity of zero. The minimumnumber of bits in a signal of interest to be read out in a seconddivided by the product of the number of desired steps in a correlationcycle, e. g., 19, the sample rate, and the signal duration, e.g., 1second, equals the maximum number of correlation cycles which should becompleted in a second, i.e., the correlation cycle rate such as 128. Thequotient of the sample rate and the correlation cycle rate equals thenumber of bits, e.g., 32, which should be loaded in a given correlationcycle so that no data signal information is lost.

The compressed output signals of the units 30 and 31 are fed throughrespective bandpass filters 50 and 51 and are applied to a balancedmodulator 52 for providing an output signal whose instantaneousfrequency is the sum of the instantaneous frequencies of the filtereddata and replica signals. The bandwidths and center frequencies of thebandpass filters 50 and 51 are selected to convert the digital pulsetrain from the units 30 and 31 into an analog signal. The sum outputsignal of the modulator 52 is fed to a bandpass filter 53. The bandwidthcharacteristics of the filters S0, 51 and 53 are similar, while thecenter frequency of the filter 53 is substantially twice that of thefilters 50 and 51.

In essence, the balanced modulator 52 and filter 53 function as a beatfrequency correlation device which can produce a constant frequencysignal if the slopes of the frequency-time characteristics of the outputsignals of the units 30 and 31 have equal magnitudes because the slopeshave opposite signs. The constant frequency signal will be stable withinabout one cycle per second if the adverse Doppler dispersion effect issubstantially eliminated, as shown above, by adjusting the relativecompression ratios of the compression units 30 and 31.

Since the Doppler effect causes a gross echo frequency shift in thereturn signal which is a function of the frequencies of the transmittedsonar signal and the relative target velocity, the output signalsupplied by the modulator 52 will be shifted in frequency if there is arelative target velocity other than zero. In order to permit the use ofa single system output filter, the output signal from the modulator 52is heterodyned with that from an oscillator output signal having afrequency which varies in steps synchronized in time with the steppingof the generators 39 and 40 for each of the various predeterminedrelative target velocities during a correlation cycle.

Accordingly, the output signal of the modulator 52 is fed through thefilter 53 to a balanced modulator 54 along with the output of a biasstep oscillator 55 which responds to the step rate clock generator 41 toprovide an output signal whose frequency varies in steps. The differenceoutput signal of the balanced modulator S4 is applied through a narrowbandwidth bandpass filter 56 and is applied to a detector and integratorunit 57 which, in turn, provides for each step a signal which is, ifdetectable, indicative of a point on the correlation function for thatstep. The output signal of the detector and integrator unit 57 isapplied to a computer and display unit 58 which is synchronized with thecontrol unit 10 and the step clock generator 41. The computer anddisplay unit 58 is mechanized to associate each correlation pointindicating signal above a minimum threshold level from the unit 57 witha particular range and with a particular relative velocity and todisplay them accordingly. Thereby, the range and relative velocity ofthe target may be tracked.

In general, it is desired that the output signal of the balancedmodulator 54 have a constant predetermined intermediate frequency.Therefore, at the time of maximum correlation the frequency of theoutput signal of the oscillator 55, f should equal the desiredintermediate frequency IF plus the frequency of the output signal of themodulator heretofore indicated to be equal to the sum of the filteredoutput frequencies of the compression units 30 and 31. Since the adverseeffects of Doppler dispersion have been corrected or alleviated by thestepped adjustment in the relative compression ratios of the units 30and 31, the center frequencies of the various signals may be used toascertain the necessary correction for the Doppler shift. The centerfrequency f d of the down-shifted data signal includes the gross Dopplershift introduced on the center frequencyfl. of the transmitted sonarsignal. Additionally, as indicated above in equation the correction forDoppler dispersion at positive relative target velocities introduces anupward frequency shift in the replica signal frequency which isequivalent to the product of the compression ratio CR the Dopplercorrection factor 2v/c, and the center frequency f of the replicasignal. For positive relative target velocities where CR is the minimum,the following relationship therefore applies: fosc IF fcd 9) fcd DX2v/c) v/C (f -XCR (Equation 7) IF CR0 [2f ZV/C (f l'f -n (Equation 8)For negative relative target velocities where CR is the minimum, thefollowing relationship applies:

fm W fcr R/ fcr (CRR) 211/0 (for) (CR /l'iZV/C (Equation 9) IF (CR/l+2v/c) z r 2% 0;, +5.91.

(Equation 10) Equations (8) and (10) may be used to ascertain theappropriate frequency of the output signal of the bias step oscillator55 for each step corresponding to a predetermined relative targetvelocity.

Since the output signal of the balanced modulator 54 should be a signalof constant frequency in those instances where the data signal iseffectively matched with the replica signal during at least one of thesteps wherein the effects of Doppler dispersion and of gross Dopplerfrequency shift are corrected, it is desirable that the filter 56 be anarrow band filter having a center frequency equal to the intermediatefrequency IF. The bandwidth of the filter 56 depends upon the desireddegree of resoltuion. For example, it has been discovered that abandwidth of twenty times the minimum compression ratio utilized in theunits 30 and 31 for a relative target velocity of zero permits analysisover a twenty cycle per second bandwidth in real time. It iscontemplated, however, that filters having a much narrower real timebandwidth such as about one cycle per second could be used which filterswould have an actual bandwidth equal to the minimum desired compressionratio. Additionally, the integrator portion of the detector andintegrator unit 57 should have an integration time set approximatelyequal to the ratio of the duration of the sonar signal being processedto the compression ratio for zero relative target velocity.

The invention therefore provides signal processing apparatus of greatflexibility which is able to detect and sort sonar return signals intoappropriate range groupings and appropriate relative target velocitygroupings. The great flexibility of the apparatus is demonstrated inthat the unit 10 need only cause a shift to the output signal of anotherof the oscillators 13 to provide multichannel operation by theapparatus. In this case, each of the channels has the same rate ofchange of frequency with time. Additionally, the apparatus is not 5limited solely to frequency modulated signals having linearly varyingfrequency-time characteristics. The apparatus disclosed may be used toprocess sonar signals characterized by more complex forms of timevarying frequency modulation. The apparatus of the invention 10 requiresthat only one replica signal need be produced which signal for thebandpass correlation type apparatus has a frequency rate of changeopposite in sign to that of the sonar signal to be transmitted. Theapparatus avoids all the problems introduced by a design apl5 proachrequiring a separate replica signal generator each for producing arespective signal having introduced thereinto the frequency shiftsimposed by Doppler effects at each corresponding relative velocity.While the invention has been described with reference to a specificembodiment suitable for sonar applications, the principles of theinvention are equally applicable to radar apparatus and the like.

What is claimed is: 1. Apparatus for processing the return signal of afrequency modulated signal transmitted through a predetermined mediumcomprising:

means for providing a data signal indicative of the frequency modulatedreturn signal; means for providing a frequency modulated replica signal;and dispersion correction means connected to receive said replica anddata signals for adjusting relatively the frequency-time characteristicsof said data and replica signals in a manner calculated to compensatefor Doppler dispersion and providing dispersion corrected data andreplica signals for correlation. 2. Apparatus according to claim 1wherein said dispersion correction means includes:

data signal and replica signal time compression means connected toreceive said data and replica signals for compressing said signals atrespective compression ratios CR and CR and providing said outputsignals suitable for correlation in time compressed form; and controlmeans connected to said compression means for adjusting relatively saidcompression ratios CR and CR in a manner calculated to correct forDoppler dispersion. 3. Apparatus according to claim 2 wherein: said timecompression means includes sampling means for sampling said data andreplica signals at the same sample rate and providing sampled data andreplica signals in digital form indicative of said data and replicasignals, and a pair of digital time compression units connected forreceiving respective ones of said sampled data and replica signals andeach being responsive to a readout-rate clock signal for providingrespective said time compressed output signals; and

said control means includes a pair of clock generators connected torespective said compression units for providing said readout-rate clocksignals of relative frequencies causing corresponding relativelyadjusted compression ratios CR and CR for said data and replica signalsfor correcting for Doppler dispersion.

4. Apparatus according to claim 2 wherein said control means includesmeans adjusting said compression ratios CR and CR relatively to oneanother in a succession of steps each calculated to correct for Dopplerdispersion introduced for a respective predetermined relative targetvelocity v in a predetermined range of velocities.

5. Apparatus according to claim 4 wherein said control means causes saidcompression ratio of said replica signal, CR to be related to that ofsaid data signal, CR in accordance with the following equation:

CR (1 2v/c) C wherein c equals the propagation velocity of thetransmitted signal through the predetermined medium.

6. Apparatus according to claim 5 wherein said control means causes saidcompression ratio CR to be related to said ratio CR for a series ofpredetermined negative relative target velocities in accordance with thefollowing equation:

CR CR /(l 2v/c).

7. Apparatus according to claim 4 wherein said control means causes saidcompression ratio of at least one of said data and replica signals toequal a predetermined minimum and causes said compression ratio of theother said signal to be adjusted in a series of steps corresponding to asuccession of predetermined relative target velocities.

8. Apparatus according to claim 4 wherein:

said control means decreases said compression ratio CR of said datasignal in a series of steps from a respective predetermined maximumvalue to a predetermined minimum value while said compression ratio CRof said replica signal has said predetermined minimum value for a seriesof successively decreased positive relative target velocities v inaccordance with the relationship:

CR (l 2v/c) CR and thereafter said control means increases saidcompression ratio CR from said predetermined minimum value to arespective predetermined maximum value while said ratio CR has saidpredetermined minimum value for a series of successively increasednegative relative velocities v in accordance with the relationship:

wherein c equals the propagation velocity of the transmitted signalthrough the predetermined medium. 9. Apparatus according to claim 1further comprising:

correlation means connected to receive said dispersion corrected dataand replica signals for correlating same and providing an output signalindicative of points on at least one correlation function. 10. Apparatusaccording to claim 9 wherein said correlation means includes:

first modulator means connected to receive said dispersion correcteddata and replica signals for providing a first modulator output signal;first oscillator means providing an output signal whose frequency isadjusted in a series of steps for each one of a series of predeterminedrelative target velocities v calculated to correct at least one saidstep of said first modulator output signal for Doppler frequency shift;

second modulator means connected to receive said first oscillator meansoutput signal and said first modulator output signal for providing asecond modulator output signal; and

filter and detector means connectd to receive said second modulatoroutput signal for providing output signals indicative of points oncorrelation functions each in a resepctive range-Doppler plane at arespective said predetermined relative target velocity.

11. Apparatus according to claim 10 wherein said 5 dispersion correctionmeans includes:

data signal and replica signal time compression means connected toreceive said data and replica signals for compressing said signals atrespective compression ratios CR and CR and providing said outputsignals suitable for correlation in time compressed form; and

control means connected to said compression means for adjustingrelatively said compression ratios CR,, and CR in a succession of stepseach calculated to correct for Doppler dispersion introduced forrespective said predetermined relative target velocities v insynchronization with corresponding said steps of output signal frequencyadjustment of said first oscillator means.

12. Apparatus according to claim 11 wherein:

said time compression means includes sampling means for sampling saiddata and replica signals at the same sample rate and providing sampleddata and replica signals in digital form indicative of said data andreplica signals, and a pair of digital time compression units connectedfor receiving respective ones of said sampled data and replica signalsand each being responsive to a readout-rate clock signal for providingrespective said time compressed output signals; and

said control means includes a step-rate oscillator for providing acorrelation cycle step control signal for each said step and a pair ofreadout-rate clock generators connected to respective said compressionunits, said generators being responsive to said cycle step controlsignals for providing in said steps said readout-rate clock signals ofrelative frequencies causing said compression units to havecorresponding relatively adjusted compression ratios CR and CR for saiddata and replica signals and said first oscillator means being connectedto receive said cycle step control signals and being responsive theretofor adjusting its said output signal frequency in said steps.

13. Apparatus according to claim 12 further comprising:

signal generator means for providing a first frequency modulated signaland said frequency modulated replica signal having a frequency-timecharacteristic equal in magnitude and opposite in sign to that of saidfirst signal; and

first mixer means for shifting the center frequency of said first signala predetermined degree upwardly to a transmission signal centerfrequency f and providing the frequency modulated signal fortransmission;

said means providing said data signal including a second mixer meansconnected to receive a signal indicative of the frequency modulatedreturn signal for shifting the center frequency thereof saidpredetermined degree downwardly and providing said data signal having acenter frequency f 14. Apparatus according to claim 13 wherein:

said first modulator means provides an output signal indicative of thesum of the frequencies of said time compressed data and replica signals;and

said oscillator means provides for each said step an output signalhaving a frequency f determined in accordance with the followingrelationship:

on: IF D fcd 2v/C (fed fc.r)]

wherein IF is a predetermined intermediate frequency, CR is thecompression ratio of said data signal and c equals the propagationvelocity of the transmitted signal through the predetermined medium.

15. Apparatus according to claim 14 wherein:

said second modulator means provides a difference output signalcomprising said second modulator output signal;

said filter and detector means includes a narrow bandwith bandpassfilter having a center frequency equal to IF; and

said control means decreases said compression ratio CR of said datasignal in a series of steps from a respective predetermined maximumvalue to a predetermined minimum value while said compression ratio CRof said replica signal has said predetermined minimum value for a seriesof successively decreased positive relative target velocities v inaccordance with the relationship:

CR (l 2v/c) CR and thereafter said control means increases saidcompression ratio CR from said predetermined minimum value to arespective predetermined maximum value while said ratio CR has saidpredetermined minimum value for a series of successively increasednegative relative velocities v in accordance with the relationship:

wherein c equals the propagation velocity of the transmitted signalthrough the predetermined medium.

16. Apparatus according to claim 15 wherein:

said readout-rate clock generators and said step-rate oscillator providetheir respective said output signals in steps corresponding to velocitysteps on the order of at most four knots.

17. Apparatus according to claim 9 further comprissaid means providingsaid frequency modulated replica signal including means for providing asaid replica signal having a varying frequency time characteristic suchthat the sum of said replica signal and the transmitted signal ofvarying frequency time characteristic yields a signal of constantfrequency.

1. Apparatus for processing the return signal of a frequency modulatedsignal transmitted through a predetermined medium comprising: means forproviding a data signal indicative of the frequency modulated returnsignal; means for providing a frequency modulated replica signal; anddispersion correction means connected to receive said replica and datasignals for adjusting relatively the frequency-time characteristics ofsaid data and replica signals in a manner calculated to compensate forDoppler dispersion and providing dispersion corrected data and replicasignals for correlation.
 2. Apparatus according to claim 1 wherein saiddispersion correction means includes: data signal and replica signaltime compression means connected to receive said data and replicasignals for compressing said signals at resPective compression ratiosCRD and CRR and providing said output signals suitable for correlationin time compressed form; and control means connected to said compressionmeans for adjusting relatively said compression ratios CRD and CRR in amanner calculated to correct for Doppler dispersion.
 3. Apparatusaccording to claim 2 wherein: said time compression means includessampling means for sampling said data and replica signals at the samesample rate and providing sampled data and replica signals in digitalform indicative of said data and replica signals, and a pair of digitaltime compression units connected for receiving respective ones of saidsampled data and replica signals and each being responsive to areadout-rate clock signal for providing respective said time compressedoutput signals; and said control means includes a pair of clockgenerators connected to respective said compression units for providingsaid readout-rate clock signals of relative frequencies causingcorresponding relatively adjusted compression ratios CRD and CRR forsaid data and replica signals for correcting for Doppler dispersion. 4.Apparatus according to claim 2 wherein said control means includes meansadjusting said compression ratios CRD and CRR relatively to one anotherin a succession of steps each calculated to correct for Dopplerdispersion introduced for a respective predetermined relative targetvelocity v in a predetermined range of velocities.
 5. Apparatusaccording to claim 4 wherein said control means causes said compressionratio of said replica signal, CRR, to be related to that of said datasignal, CRD, in accordance with the following equation: CRR (1 + 2v/c)CD wherein c equals the propagation velocity of the transmitted signalthrough the predetermined medium.
 6. Apparatus according to claim 5wherein said control means causes said compression ratio CRD to berelated to said ratio CRR for a series of predetermined negativerelative target velocities in accordance with the following equation:CRD CRR/(1 + 2v/c).
 7. Apparatus according to claim 4 wherein saidcontrol means causes said compression ratio of at least one of said dataand replica signals to equal a predetermined minimum and causes saidcompression ratio of the other said signal to be adjusted in a series ofsteps corresponding to a succession of predetermined relative targetvelocities.
 8. Apparatus according to claim 4 wherein: said controlmeans decreases said compression ratio CRD of said data signal in aseries of steps from a respective predetermined maximum value to apredetermined minimum value while said compression ratio CRR of saidreplica signal has said predetermined minimum value for a series ofsuccessively decreased positive relative target velocities v inaccordance with the relationship: CRD (1 + 2v/c) CRR and thereafter saidcontrol means increases said compression ratio CRR from saidpredetermined minimum value to a respective predetermined maximum valuewhile said ratio CRD has said predetermined minimum value for a seriesof successively increased negative relative velocities v in accordancewith the relationship: CRR CRD/(1 + 2v/c) wherein c equals thepropagation velocity of the transmitted signal through the predeterminedmedium.
 9. Apparatus according to claim 1 further comprising:correlation means connected to receive said dispersion corrected dataand replica signals for correlating same and providing an output signalindicative of points on at least one correlation function.
 10. Apparatusaccording to claim 9 wherein said correlation means includes: firstmodulator means connected tO receive said dispersion corrected data andreplica signals for providing a first modulator output signal; firstoscillator means providing an output signal whose frequency is adjustedin a series of steps for each one of a series of predetermined relativetarget velocities v calculated to correct at least one said step of saidfirst modulator output signal for Doppler frequency shift; secondmodulator means connected to receive said first oscillator means outputsignal and said first modulator output signal for providing a secondmodulator output signal; and filter and detector means connectd toreceive said second modulator output signal for providing output signalsindicative of points on correlation functions each in a resepctiverange-Doppler plane at a respective said predetermined relative targetvelocity.
 11. Apparatus according to claim 10 wherein said dispersioncorrection means includes: data signal and replica signal timecompression means connected to receive said data and replica signals forcompressing said signals at respective compression ratios CRD and CRRand providing said output signals suitable for correlation in timecompressed form; and control means connected to said compression meansfor adjusting relatively said compression ratios CRD and CRR in asuccession of steps each calculated to correct for Doppler dispersionintroduced for respective said predetermined relative target velocitiesv in synchronization with corresponding said steps of output signalfrequency adjustment of said first oscillator means.
 12. Apparatusaccording to claim 11 wherein: said time compression means includessampling means for sampling said data and replica signals at the samesample rate and providing sampled data and replica signals in digitalform indicative of said data and replica signals, and a pair of digitaltime compression units connected for receiving respective ones of saidsampled data and replica signals and each being responsive to areadout-rate clock signal for providing respective said time compressedoutput signals; and said control means includes a step-rate oscillatorfor providing a correlation cycle step control signal for each said stepand a pair of readout-rate clock generators connected to respective saidcompression units, said generators being responsive to said cycle stepcontrol signals for providing in said steps said readout-rate clocksignals of relative frequencies causing said compression units to havecorresponding relatively adjusted compression ratios CRD and CRR forsaid data and replica signals and said first oscillator means beingconnected to receive said cycle step control signals and beingresponsive thereto for adjusting its said output signal frequency insaid steps.
 13. Apparatus according to claim 12 further comprising:signal generator means for providing a first frequency modulated signaland said frequency modulated replica signal having a frequency-timecharacteristic equal in magnitude and opposite in sign to that of saidfirst signal; and first mixer means for shifting the center frequency ofsaid first signal a predetermined degree upwardly to a transmissionsignal center frequency fcx and providing the frequency modulated signalfor transmission; said means providing said data signal including asecond mixer means connected to receive a signal indicative of thefrequency modulated return signal for shifting the center frequencythereof said predetermined degree downwardly and providing said datasignal having a center frequency fcd.
 14. Apparatus according to claim13 wherein: said first modulator means provides an output signalindicative of the sum of the frequencies of said time compressed dataand replica signals; and said oscillator means provides for each saidstep an output signal having a frequency fosc determined in accordancewith the follOwing relationship: fosc IF + CRD (2fcd + 2v/c (fcd + fcx))wherein IF is a predetermined intermediate frequency, CRD is thecompression ratio of said data signal and c equals the propagationvelocity of the transmitted signal through the predetermined medium. 15.Apparatus according to claim 14 wherein: said second modulator meansprovides a difference output signal comprising said second modulatoroutput signal; said filter and detector means includes a narrow bandwithbandpass filter having a center frequency equal to IF; and said controlmeans decreases said compression ratio CRD of said data signal in aseries of steps from a respective predetermined maximum value to apredetermined minimum value while said compression ratio CRR of saidreplica signal has said predetermined minimum value for a series ofsuccessively decreased positive relative target velocities v inaccordance with the relationship: CRD (1 + 2v/c) CRR and thereafter saidcontrol means increases said compression ratio CRR from saidpredetermined minimum value to a respective predetermined maximum valuewhile said ratio CRD has said predetermined minimum value for a seriesof successively increased negative relative velocities v in accordancewith the relationship: CRR CRD/(1 + 2v/c) wherein c equals thepropagation velocity of the transmitted signal through the predeterminedmedium.
 16. Apparatus according to claim 15 wherein: said readout-rateclock generators and said step-rate oscillator provide their respectivesaid output signals in steps corresponding to velocity steps on theorder of at most four knots.
 17. Apparatus according to claim 9 furthercomprising: said means providing said frequency modulated replica signalincluding means for providing a said replica signal having a varyingfrequency time characteristic such that the sum of said replica signaland the transmitted signal of varying frequency time characteristicyields a signal of constant frequency.